The invention relates generally to the field of cytogenetics, and more particularly, to methods for identifying and classifying chromosomes.
Chromosome abnormalities are associated with genetic disorders, degenerative diseases, and exposure to agents known to cause degenerative diseases, particularly cancer, German, xe2x80x9cStudying Human Chromosomes Today,xe2x80x9d American Scientist, Vol. 58, pgs. 182-201 (1970); Yunis, xe2x80x9cThe Chromosomal Basis of Human Neoplasia,xe2x80x9d Science, Vol. 221, pgs. 227-236 (1983); and German, xe2x80x9cClinical Implication of Chromosome Breakage,xe2x80x9d in Genetic Damage in Man Caused by Environmental Agents, Berg, Ed., pgs. 65-86 (Academic Press, New York, 1979). Chromosomal abnormalities can be of three general types: extra or missing individual chromosomes, extra or missing portions of a chromosome, or chromosomal rearrangements. The third category includes translocations (transfer of a piece from one chromosome onto another chromosome), and inversions (reversal in polarity of a chromosomal segment).
Detectable chromosomal abnormalities occur with a frequency of one in every 250 human births. Abnormalities that involve deletions or additions of chromosomal material alter the gene balance of an organism and generally lead to fetal death or to serious mental physical defects. Down""s syndrome is caused by having three copies of chromosome 21 instead of the normal 2. This syndrome is an example of a condition caused by abnormal chromosome number, or aneuploidy. Chronic myelogeneous leukemia is associated with the exchange of chromosomal material between chromosome 9 and chromosome 22. The transfer of chromosomal material in this leukemia is an example of a translocation. Clearly, a sensitive method for detecting chromosomal abnormalities would be a highly useful tool for genetic screening.
Measures of chromosome breakage and other aberrations caused by ionizing radiation or chemical mutagens are widely used as quantitative indicators of genetic damage caused by such agents, Biochemical Indicators of Radiation Injury in Man (International Atomic Energy Agency, Vienna, 1971); and Berg, Ed. Genetic Damage in Man Caused by Environmental Agents (Academic Press, New York, 1979). A host of potentially carcinogenic and teratogenic chemicals are widely distributed in the environment because of industrial and agricultural activity. These chemicals include pesticides, and a range of industrial wastes and by-products, such as halogenated hydrocarbons, vinyl chloride, benzene, arsenic, and the like, Kraybill et al., Eds., Environmental Cancer (Hermisphere Publishing Corporation, New York, 1977). Sensitive measures of chromosomal breaks and other abnormalities could form the basis of improved dosimetric and risk assessment methodologies for evaluating the consequences of exposure to such occupational and environmental agents.
Current procedures for genetic screening and biological dosimetry involve the analysis of karyotypes. A karyotype is a collection of indices which characterize the state of an organism""s chromosomal complement. It includes Such things as total chromosome number, copy number of individual chromosome types (e.g., the number of copies of chromosome X), and chromosomal morphology, e.g., as measured by length, centromeric index, connectedness, or the like. Chromosomal abnormalities can be detected by examination of karyotypes. Karyotypes are determined by staining an organism""s metaphase, or condensed, chromosomes. Metaphase chromosomes are used because, until recently, it has not been possible to visualize nonmetaphase, or interphase chromosomes due to their dispersed condition in the cell nucleus.
The metaphase chromosomes can be stained by a number of cytological techniques to reveal a longitudinal segmentation into entities generally referred to as bands. The banding pattern of each chromosome within an organism is unique, permitting unambiguous chromosome identification regardless of morphological similarity, Latt, xe2x80x9cOptical Studies of Metaphase Chromosome Organization,xe2x80x9d Annual Review of Biophysics and Bioengineering, Vol. 5, pgs. 1-37 (1976). Adequate karyotyping for detecting some important chromosomal abnormalities, Such as translocations and inversions requires banding analysis. Unfortunately, such analysis requires cell culturing and preparation of high quality metaphase spreads, which is extremely difficult and time consuming, and almost impossible for tumor cells.
The sensitivity and resolving power of current methods of karyotyping, are limited by the lack of stains that can readily distinguish different chromosomes having highly similar staining characteristics because of similarities in such gross features as size, morphology, and/or DNA base composition.
In recent years rapid advances have taken place in the study of chromosome structure and its relation to genetic content and DNA composition. In part, the progress has come in the form of improved methods of gene mapping based on the availability of large quantities of pure DNA and RNA fragments for probes produced by genetic engineering techniques, e.g., Kao, xe2x80x9cSomatic Cell Genetics and Gene Mappings,xe2x80x9d International Review of Cytology, Vol. 85, pgs. 109-146 (1983), and D""Eustacnio et al., xe2x80x9cSomatic Cell Genetics in Gene Families,xe2x80x9d Science, Vol. 220, pgs. 9, 19-924 (1983). The probes for gene mapping comprise labeled fragments of single stranded or double stranded DNA or RNA which are hybridized to complementary sites on chromosomal DNA. The following references are representative of studies utilizing gene probes for mapping: Harper et al. xe2x80x9cLocalization of the Human Insulin Gene to the Distal End of the Short Arm of Chromosome 11,xe2x80x9d Proc. Natl. Acad. Sci., Vol. 78, pgs. 4458-4460; Kao et al., xe2x80x9cAssignment of the Structural Gene Coding for Albumin to Chromosome 4,xe2x80x9d Human Genetics, Vol. 62, pgs. 337-341 (1982); Willard et al., xe2x80x9cIsolation and Characterization of a Major Tandem Repeat Family from the Human X Chromosome,xe2x80x9d Nucleic Acids Research, Vol. 11, pgs. 2017-2033 (1983); and Falkow et al., U.S. Pat. No. 4,358,535, issued Nov. 9, 1982, entitled xe2x80x9cSpecific DNA Probes in Diagnostic Microbiology.xe2x80x9d The hybridization process involves unravelling, or melting, the double stranded nucleic acids by heating, or other means. This step in the hybridization process is sometimes referred to as denaturing the nucleic acid. When the mixture of probe and target nucleic acids cool, strands having complementary bases recombine, or anneal. When a probe anneals with a target nucleic acid, the probe""s location on the target can be detected by a label carried by the probe. When the target nucleic acid remains in its natural biological setting, e.g., DNA in chromosomes or cell nuclei (albeit fixed or altered by preparative techniques) the hybridization process is referred as in situ hybridization.
Use of hybridization probes has been limited to identifying the location of genes or known DNA sequences on chromosomes. To this end it has been crucially important to produce pure, or homogeneous, probes to minimize hybridizations at locations other than at the site of interest, Henderson, xe2x80x9cCytological Hybridization to Mammalian Chromosomes,xe2x80x9d International Review of Cytology, Vol. 76, pgs. 1-46 (1982).
Manuelidis et al., in xe2x80x9cChromosomal and Nuclear Distribution of the Hind III 1.9-KB Human DNA Repeat Segment,xe2x80x9d Chromosoma, Vol. 91, pgs. 28-38 (1984), disclose the construction of a single kind of DNA probe for detecting multiple loci on chromosomes corresponding to members of a family of repeated DNA sequences.
Wallace et al., in xe2x80x9cThe Use of Synthetic Oligonucleotides as Hybridization Probes. II. Hybridization of Oligonucleotides of Mixed Sequence to Rabbit Beta-Globin DNA, xe2x80x9cNucleic Acids Research, Vol. 9, pgs. 879-894 (1981), disclose the construction of synthetic oligonucleotide probes having mixed base sequences for detecting a single locus corresponding to a structural gene. The mixture of base sequences was determined by considering all possible nucleotide sequences which could code for a selected sequence of amino acids in the protein to which the structural gene corresponded.
Olsen et al., in xe2x80x9cIsolation of Unique Sequence Human X Chromosomal Deoxyribonucleic Acid,xe2x80x9d Biochemistry, Vol. 19, pgs. 2419-2428 (1980), disclose a method for isolating labeled unique sequence human X chromosomal DNA by successive hybridizations: first, total genomic human DNA against itself so that a unique sequence DNA fraction can be isolated; second, the isolated unique sequence human DNA fraction against mouse DNA so that homologous mouse/human sequences are removed; and finally, the unique sequence human DNA not homologous to mouse against the total genomic DNA of a human/mouse hybrid whose only human chromosome is chromosome X, so that a fraction of unique sequence X chromosomal DNA is isolated.
The invention includes methods and compositions for staining chromosomes. In particular, chromosome specific staining reagents are provided which comprise heterogeneous mixtures of labeled nucleic acid fragments having substantial portions of substantially complementary base sequences to the chromosomal DNA for which specific staining is desired. The nucleic acid fragments of the heterogenous mixtures include double stranded or single stranded RNA or DNA. Heterogeneous in reference to the mixture of labeled nucleic acid fragments means that the staining reagents comprise many copies each of fragments having different base compositions and/or sizes, Such that application of the staining reagent to a chromosome results in a substantially uniform distribution of fragments hybridized to the chromosomal DNA.
xe2x80x9cSubstantial proportionsxe2x80x9d in reference to the basic sequences of nucleic acid fragments that are complementary to chromosomal DNA means that the complementarity is extensive enough so that the fragments form stable hybrids with the chromosomal DNA under standard hybridization conditions for the size and complexity of the fragment. In particular, the term comprehends the situation where the nucleic acid fragments of the heterogeneous mixture possess regions having non-complementary base sequences.
As discussed more fully below, preferably the heterogeneous mixtures are substantially free from so-called repetitive sequences, both the tandem variety and the interspersed variety (see Hood et al., Molecular Biology of Eucaryotic Cells (Benjamin/Cummings Publishing Company, Menlo Park, Calif., 1975) for an explanation of repetitive sequences). Tandem repeats are so named because they are clustered or contiguous on the DNA molecule which forms the backbone of a chromosome. Members of this class of repeats are also associated with well-defined regions of the chromosome, e.g., the centromeric region. Thus, if these repeats form a sizable fraction of a chromosome, and are removed from the heterogeneous mixture of fragments employed in the invention, perfect uniformity of staining may not be possible. This situation is comprehended by the use of the term xe2x80x9csubstantially uniformxe2x80x9d in reference to the heterogeneous mixture of labeled nucleic acid fragments of the invention.
It is desirable to disable the hybridization capacity of repetitive sequences because copies occur on all the chromosomes of a particular organism; thus, their presence reduces the chromosome specificity of the staining reagents of the invention. As discussed more fully below, hybridization capacity can be disabled in several ways, e.g., selective removal or screening of repetitive sequences from chromosome specific DNA, selective blocking of repetitive sequences by pre-reassociation with complementary fragments, or the like.
Preferably, the staining reagents of the invention are applied to interphase or metaphase chromosomal DNA by in situ hybridization, and the chromosomes are identified or classified, i.e., karyotyped, by detecting the presence of the label on the nucleic acid fragments comprising the staining reagent.
The invention includes chromosome staining reagents for the total genomic complement of chromosomes, staining reagents specific to single chromosomes, staining reagents specific to subsets of chromosomes, and staining reagents specific to subregions within a single chromosome. The term xe2x80x9cchromosome-specific,xe2x80x9d is understood to encompass all of these embodiments of the staining reagents of the invention. The term is also understood to encompass staining reagents made from both normal and abnormal chromosome types.
A preferred method of making the staining reagents of the invention includes isolating chromosome-specific DNA, cloning fragments of the isolated chromosome-specific DNA to form a heterogeneous mixture of nucleic acid fragments, disabling the hybridization capacity of repeated sequences in the nucleic acid fragments, and labeling the nucleic acid fragments to form a heterogeneous mixture of labeled nucleic acid fragments. As described more fully below, the ordering of the steps for particular embodiments varies according to the particular means adopted for carrying out the steps.
The preferred method of isolating chromosome-specific DNA for cloning includes isolating specific chromosome types by fluorescence-activated sorting.
The present invention addresses problems associated with karyotyping chromosomes, especially for diagnostic and dosimetric applications. In particular, the invention overcomes problems which arise because of the lack of stains that are sufficiently chromosome-specific by providing reagents comprising heterogeneous mixtures of labeled nucleic acid fragments that can be hybridized to the DNA of specific chromosomes, specific subsets of chromosomes, or specific subregions of specific chromosomes. The staining technique of the invention opens up the possibility of rapid and highly sensitive detection of chromosomal abnormalities in both metaphase and interphase cells using standard clinical and laboratory equipment. It has direct application in genetic screening, cancer diagnosis, and biological dosimetry.